A typical conventional integrated antenna system (such as that used in a phased array antenna) may include many radiating elements. Referring to FIG. 1, each radiating element 110 may comprise of the following components: radiator 110 (including a matching taper 113 and balun 116), an optional filter 120, circulator 130, and high power amplifier (HPA) 140. (For simplicity of illustration, the feed network is omitted.) All these components are designed separately, yet to a common interface impedance (for example, 50 ohm), including the radiator input balun 116. The radiator output impedance, however, is necessarily different: free space wave impedance is typically about 377 ohms. The typical constraint on the system impedance (i.e., to 50 ohms) coupled with the need to radiate into free space at 377 ohms drastically degrades transmitter performance and increase the weight and volume of the system. Transmitters designed based on the aforementioned approach suffer from low efficiency, high prime power requirement, large volume, and weight.
In an active electronically scanned array (AESA), in particular, where the need for high integration and small form factor is especially acute, the impedance match problem can have serious consequence. For example, lower power efficiency leads to undesirably high thermal loads and increased prime power requirements. High internal impedance mismatch can also limit bandwidth, in accordance with Fano's theorem, and the use of longer radiators. Impedance mismatch also can result in lowered reverse isolation, which causes amplifier load pull and reduced stability. Finally, the high distortion resulting from internal reflections degrades pulse-on-pulse and pulse-on-continuous wave (CW) performance of the radar.
One conventional approach in commercial wireless radio frequency (RF) applications is to use integrated active antennas that attempt to better match the internal impedances, thereby enhancing efficiency. Such antennas typically employ a longer radiator impedance taper to reduce impedance mismatch, but this reduces the antenna bandwidth and increases the physical size of the antenna. However, such integrated active antennas have not, to date, proved to be viable solutions for microwave phased array applications.